Near-Field Microscopy Probes Quantum Dots

Near-field optical microscopy is enabling researchers at the University of Michigan in Ann Arbor and at the Naval Research Laboratory in Washington to investigate the fundamental physical characteristics of quantum dots. The work will lead to a better understanding of the inner workings of quantum dots and may open the door to computers based on quantum-dot technology.

Near-field optical microscopy has enabled researchers to perform a spectroscopic analysis of quantum dots. In this volumetric representation of the nonlinear data, small signals are translucent, and large signals -- quantum states of the excitons -- are opaque. Courtesy of Jeffrey R. Guest.A quantum dot is any structure where the spatial dimensions have a detectable impact on the quantum features of the system," said Duncan G. Steel, Peter S. Fuss professor of electrical engineering and computer science and professor of physics at the university. For example, structures in GaAs as small as 1000 Å can alter the optical and electronic properties of the semiconductor.

Stephan W. Koch, who is conducting similar research with a different group at Philipps Universität Mar-burg in Germany, explained that while spectroscopy offers information about the quantum state of a particular dot, quantum dots often are smaller than the minimum beam waist of the probe. As a result, scientists must average the characteristics of large numbers of dots. Unfortunately, to use them in real-world applications, researchers need information about individual dots.

Near-field optical microscopy offers a solution. In the technique, a tapered optical probe that is smaller than the wavelength of visible light can break the resolution limit. As long as the probe remains very close to the sample, the light sent down it in the near field may be used for spectroscopic analysis and for conventional optical images.

Observation of level repulsion

Steel said that even though these probes are larger than a quantum dot, it is possible to investigate individual dots if the resolution is less than the interdot spacing.

Jeffrey R. Guest, a postdoctoral fellow at the University of Michigan, explained that the researchers designed and constructed a near-field microscope that operated at 4 K so that they could remove thermal energy from the quantum-dot system and reduce phonon-induced broadening. The combined output of two frequency-locked continuous-wave lasers served as the excitation source, and the scientists collected the spectra of the dots either with a built-in photodiode or with an avalanche photodiode at room temperature.

Steel said the work has enabled the first observation of level repulsion. "At the quantum level, a complex landscape in the sample leads to energy states avoiding each other if they are close together spatially," he explained. "This is a somewhat esoteric quantum phenomenon; however, the work opens up the way to carry out these studies on single molecules."

The researchers hope to make the system easier to use and more sensitive to weak optical coupling, he said, and the technique may have applications in nanotechnology. "We are continuing with our main work on quantum computing ... and this system will be useful in demonstrating specific features of quantum logic devices."